The invention generally relates to metal-oxygen-carbon field emitters and particularly to their use in a field emitter cathode for display panels. A process for making metal-oxygen-carbon whisker field emitters is also provided.
Field emission electron sources, often referred to as field emission materials or field emitters, can be used in a variety of electronic applications, e.g., vacuum electronic devices, flat panel computer and television displays, emission gate amplifiers, and klystrons and in lighting.
Display panels are used in a wide variety of applications such as home and commercial televisions, laptop and desktop computers and indoor and outdoor advertising and information presentations. Flat panel displays are only a few inches thick in contrast to the deep cathode ray tube monitors found on most televisions and desktop computers. Flat panel displays are a necessity for laptop computers, but also provide advantages in weight and size for many of the other applications. Currently, laptop computer flat panel displays use liquid crystals which can be switched from a transparent state to an opaque one by the application of small electrical signals. It is difficult to reliably produce these displays in sizes larger than that suitable for laptop computers.
Plasma displays have been proposed as an alternative to liquid crystal displays. A plasma display uses tiny cells of electrically charged gases to produce an image and requires relatively large electrical power to operate.
Flat panel displays having a cathode using a field emission electron source, i.e., a field emission material or field emitter, and a phosphor capable of emitting light upon bombardment by electrons emitted by the field emitter have been proposed. Such displays have the potential for providing the visual display advantages of the conventional cathode ray tube and the depth, weight and power consumption advantages of the other flat panel displays. U.S. Pat. Nos. 4,857,799 and 5,015,912 disclose matrix-addressed flat panel displays using micro-tip cathodes constructed of tungsten, molybdenum or silicon. WO 94-15352, WO 94-15350 and WO 94-28571 disclose flat panel displays wherein the cathodes have relatively flat emission surfaces.
R. S. Robinson et al., J. Vac. Sci. Technolo. 21 (3), 790 (1982) disclose the formation of cones on the surfaces of substrates under ion bombardment. The effect was reported for various substrate materials and the cones were generated by simultaneously sputtering a surface at high energy while seeding it with impurity atoms deposited at low energy. They also disclosed the formation of carbon whiskers up to 50 xcexcm in length when a graphite substrate was ion-bombarded with impurities from a stainless steel target.
J. A. Floro, S. M. Rossnagel, and R. S. Robinson, J. Vac. Sci. Technolo. A 1 (30), 1398 (1983) disclose the formation of whiskers during relatively high current density ion bombardment of heated graphite substrates. The whiskers were disclosed to be 2-50 xcexcm in length and 0.05-0.5 xcexcm in diameter and to grow parallel to the ion beam. Simultaneous impurity seeding was reported to inhibit whisker growth.
J. A. van Vechten, W. Solberg, P. E. Batson, J. J. Cuomo, and S. M. Rossnagel, J. Crystal Growth 82, 289 (1987) discuss the growth of whiskers from graphite surfaces under ion sputtering conditions. They note that the whiskers of smallest diameter, characteristically about 15 nm, definitely appear to be different from either diamond or the scrolled-graphite structure found in carbon fibers grown by catalytic pyrolysis of hydrocarbons. Larger whiskers with diameters ranging from 30 to 100 nm were also observed to grow in sputtering systems. The smaller diameter whiskers are constant in diameter along the length while the larger diameter whiskers may have a slight taper.
M. S. Dresselhaus, G. Dresselhaus, K. Sugihara, I. L. Spain, and H. A. Goldberg, Graphite Fibers and Filaments (Springer-Verlag, Berlin, 1988), pp. 32-34, disclose that filaments may be grown on several types of hexagonal carbon surfaces, but not on diamond or glassy carbon.
In view of the above, improved field emission materials are needed for use in field emitter cathodes for display panels and other electronic devices. Other objects and advantages of the present invention will become apparent to those skilled in the art upon reference to the attached drawings and to the detailed description of the invention which hereinafter follows.
The invention provides a field emission electron emitter composition comprising metal, oxygen and carbon, wherein the atomic ratio of metal:oxygen:carbon is a:b:c where a is from about 0.1 to about 0.4, b is from about 0.1 to about 0.8 and c is from about 0.05 to about 0.8 with the proviso that a+b+c=1.
The invention also provides a field emission cathode comprising a metal, oxygen and carbon field emission electron emitter composition, as described above, attached to a metal substrate. Preferably, the field emission electron emitter composition is in the form of whiskers. Preferably, a is from about 0.2 to about 0.4, b is from about 0.4 to about 0.8 and c is from about 0.05 to about 0.3. Preferably the metal is tungsten, iron or molybdenum.
A process for making the metal-oxygen-carbon whiskers of the invention is also provided. The process comprises heating a metal substrate, preferably a metal wire, coated with an organic polymer, such as polyacrylonitrile (PAN), to a temperature of from about 1100xc2x0 C. to about 1550xc2x0 C. in an inert atmosphere and maintaining that temperature for about 15 minutes to about two (2) hours. Preferably, the temperature is from about 1150xc2x0 C. to about 1300xc2x0 C. and the atmosphere comprises argon. When the heating is carried out in the presence of a catalyst, temperatures as low as 550xc2x0 C. can be used with heating times up to three (3) hours. Suitable catalysts include nickel, copper-nickel alloys and cobalt-nickel alloys.
The field emission electron emitter compositions and field emission cathodes are useful in vacuum electronic devices, flat panel computer and television displays and other large screen applications, emission gate amplifiers, klystrons and lighting devices. As used herein, the term xe2x80x9cdisplay panelxe2x80x9d embraces planar (e.g., flat panel displays) and curved surfaces as well as other possible geometries).